1. Field of the Invention
The present invention relates to a stereoscopic retinal camera, and more particularly to a focus detecting system to be equipped in a stereoscopic retinal camera.
2. Description of Related Art
In focusing a conventional retinal camera on the fundus of an examinee's eye, a linear reticule of an observing system is superposed on a spatial image of the retina, and the focusing condition of the spatial image is observed by the observing system.
A monocular retinal camera has been proposed in Japanese Patent Publication No. SHO 61(1986)-39050, in which an index projecting system is disposed in an optical path branched from an optical path of a photographing optical system.
Referring to FIG. 11, the monocular retinal camera is provided with a light path dividing means disposed behind a perforated mirror and an index projecting system disposed in the optical path branched by the light path dividing means, by which an image of an index is projected to the fundus of the eye.
More specifically, an index projecting system C is disposed in the optical path branched from an optical path of a photographing optical system A by a reflecting mirror 45 which is the light path dividing means, whereby the focusing operation of the retinal camera on the fundus of the examinee's eye is not affected by illuminating condition, the index projecting system C and the photographing optical system A may be simply interlocked so that both focal parts thereof are moved together. In the camera, index projecting system C projects an infrared light beam on the fundus E.sub.R of the eye without using mydriatica or the like to the eye, and whether the infrared image of the index is in or out of focus on the fundus E.sub.R is judged. Accordingly, the alignment of the camera to the fundus of the eye can be achieved with a simple and easy mechanism.
Japanese Patent Publication No. SHO 53(1978)-43277 also discloses another monocular retinal camera in which an index projecting system is disposed in an optical path branched from an optical path of an illumination optical system.
Referring to FIG. 12, the retinal camera is provided with a photographing system A for photographing an image of a fundus E.sub.R of an eye, an illumination system B for irradiating an infrared beam on the fundus E.sub.R via a light path dividing means 55 arranged in the optical path of the photographing system A, a focus index projecting system C for projecting a plurality of focusing infrared beams emerging from a focus index 60 on the fundus E.sub.R, which is movable in relation with the photographing system A, and an observing system D including an infrared image display means 65 in which an infrared light image reflected by the fundus is converted into a visible image, each system B, C and D using the optical path of the photographing system A in common.
In the retinal camera, a fundus image and a focusing index image formed by the both infrared light beams, reflected by the fundus are converted into visible images through the observing system. Then the photographing system A is moved so as to change a direction of the optical axis relatively to the fundus to determine a photographing visible field while the fundus is observed, and the focusing index projecting system C interlocking with the photographing system A is adjusted so that the plurality of focus index images coincide with one shape focus index image, accordingly, the photographing system A may be focused on the fundus of the eye.
However, in the former retinal camera, the observing condition, i.e., the condition of the image, is directly dependent on the diopter, i.e., the refracting power of the eyes of the observer, accordingly, the refracting power of a view finder should be exactly adjusted and precise alignment needs highly skillful and experienced technique.
In the latter retinal camera, the index projecting light beam passing through the space between the illuminating light beam and the photographing light beam or overlap part of the illuminating light beam, which makes adjustment of the optical system difficult. And, it is also difficult to manufacture and adjust the retinal camera because the light path dividing means for the index projecting system has to be disposed so as not to eclipse the optical path of a focus lens in the photographing optical system. Additionally, if the construction is applied to a stereoscopic retinal camera which needs two photographing luminous flux, it is foreseen that its constitution may become too complex.
Further, the latter retinal camera is applied to a monocular retinal camera, alternatively in a case that the camera is applied to a stereoscopic retinal camera to obtain two picture images having different parallax in right and left pictures, the following problem may be occurred. Namely, two photographing optical axes, for photograph of a fundus with the stereoscopic retinal camera, are often in eccentricity different from a cornea and a crystalline lens, accordingly, the latter retinal camera as it is may not obtain stereoscopic image of the fundus to be observed by the stereoscopic retinal camera. There is no idea about how the stereoscopic optical system may be utilized in the latter retinal camera, and in what focussing condition each of right and left stereoscopic images should be, accordingly, adaptability of the latter construction to the stereoscopic retinal camera is not sufficient.